When used in a RMN probe, the resonator of the invention is described in detail in the French patents FR-A-1 447 226 and FR-A-2 098 624 and accordingly this device shall not be described here in further detail. It merely suffices to mention that the probe includes one or several flasks containing a liquid sample (also called a "radical solution"), these flasks being disposed in a coaxial resonator. This resonator is constituted by a central conductor traversing the flask(s) and an external conductor situated around the flask(s). The probe includes windings for taking up and reinjecting a signal on the LARMOR frequency. This frequency is defined by the magnetic field in which the probe is immersed and by the actual gyromagnetic ratio to the liquid sample used.
An RMN resonator according to the prior art is shown on the annexed FIG. 1 and includes:
a central conductor 10 having the shape of a circular cylinder with an axis A with one first extremity 12 and one second extremity 14; PA1 an external conductor 20 rotating around the axis A and constituted by a conductive film deposited on the outer wall of the flasks 22 containing the radical solution, the conductive film generally being divided into sectors; PA1 tuning capacitors 24 connected between the first extremity 12 of the central conductor 10 and the external conductor 20. PA1 it is difficult to find capacitors offering all the conditions required in this application, namely good quality at high frequencies, absence of any rectifier effect, good voltage behaviour, nonmagnetism, ease of adjustment, etc.; PA1 strong radiation is observed at high frequencies at the level of the capacitors, unless these are shielded; PA1 the expansion bubble, which inevitably occurs in flasks, may be placed at the level of the adjustment capacitors and detune the resonator; PA1 the differences (about .+-.10%) between the capacities of the various tuning capacitors result in differences of intensity in the currents circulating in the sectors of the external conductor to which they are connected and, accordingly, results in losses by radiation. PA1 an embodiment of extremely reliable resonators (high frequency quality, absence of any rectifier effect, good voltage behaviour, non-magnetism) is simple; as regards adjustment, this is simply obtained by sliding one armature with respect to the other; PA1 radiation at the end of the resonator is suppressed; PA1 detuning due to displacement of the expansion bubble is avoided since the bubbles affects the value of the capacitor independent of its position; PA1 the equality (to within 0.5%) between the various partial capacities of the distributed capacity ensures symmetry of the currents circulating in the sectors of the external conductor and accordingly an absence of any radiation.
This resonator is fed by a coaxial cable 30 having a central core 32 and an external conductive sheath 34, such as a braid; the sheath is connected to the external conductor 20 and the core 32 to the second extremity; in addition, this extremity 14 is connected in turn to the braid by a loop 38, generally constituted by a silver wire.
This resonator functions as follows. The radiofrequency energy is brought by the coaxial cable 30. The resonance frequency is adjusted by the capacitors 24. The central conductor 10 constitutes a "hot" point (from the point of view of the potential) and the external conductor 20 constitutes a "cold" point. The impedance adaptation between the coaxial cable (whose impedance is generally 50 Ohms) and the resonator is obtained by the loop 38, which behaves like an adjustable inductive resistor short-circuit disposed at the extremity of the coaxial cable.
The equivalent circuit diagram of the unit is shown on the accompanying FIG. 2. On the section (a), this diagram shows a resonator 40 with an adaptation loop 42 and a tuning capacitor 44. The resonator 40 is disposed with impedances distributed along the axis z. This means that between the dimension z and the dimension z+dz, an elementary section of the resonator is equivalent to the circuit of the section (b) with two inductive resistors L/2, two resistors R/2, one capacitor C with one parallel resistor R', the values L, C and R, R' being functions of the geometry of the resonator (therefore of z) and the dielectric elements it contains.
Although satisfactory in certain respects, such resonators exhibit a certain number of drawbacks mainly linked to the presence of an adjustment capacitor or capacitors (24 on FIG. 1 and 44 on the diagram of FIG. 2a). These drawbacks are the following: